We have received several phone calls and Plant Clinic samples of soybean plants that are demonstrating "unusual" symptoms, usually described as "puckering." While the 2000 growing season has been relatively favorable for soybean development and has also allowed for timely applications of postemergence herbicides, the leaf-puckering phenomenon has again appeared. This soybean growth abnormality has appeared during several previous growing seasons and was somewhat common and widely distributed during the 1997, 1998, and 1999 growing seasons. The 1997 season was not the first season the problem was observed, but it may have been as widespread during 1997 as any in recent memory. This phenomenon is not restricted to soybean fields in Illinois; weed scientists in several midwestern states have also reported instances of puckered soybeans. The symptoms that have frequently been reported include the following: |
1. Extreme cupping of trifoliolate leaves, usually most pronounced on the upper trifoliolates.
2. Veins of affected leaves tend to assume a parallel orientation instead of the usual net veination pattern.
3. Tips of cupped leaves with parallel veins are often brown.
4. Plants are stunted as compared to plants not demonstrating the aforementioned symptoms; these plants may remain stunted the remainder of the season, but this does not always happen.
The most difficult issue to determine with respect to puckered soybeans is identifying the cause or causal agent. Several theories have been put forward by weed scientists across several states in the north central region and are presented here. Note that it is very unlikely that only one of these possibilities will explain the cause of puckered soybeans in all instances.
1. Somehow, the plants have been exposed to a growth regulator herbicide.
The growth regulator herbicides tend to mimic the effects of endogenous plant hormones, in particular auxins. Plant hormones control many growth and development processes affecting the growth of the plant. These hormones are physiologically active with-in the plant at extremely low concentrations (parts per million or billion); exposing a soybean plant to a synthetic type of hormone (i.e., a plant growth regulator herbicide) can induce a wide range of responses within the plant, ranging from slight morphological modifications (leaf abnormalities, for example) to plant death. The degree or severity of response is partially dependent on the concentration of herbicide the plant was exposed to as well as environmental conditions and crop variety. The literature has many references to research conducted on the response of various crops to exposure of sublethal amounts of various growth regulator herbicides. Most of these studies were conducted more than 20 years ago, but the symptoms of exposure these studies describe are very similar to those encountered during this and previous growing seasons.
How much (concentration) growth regulator does it take to induce symptoms? Dicotyledonous plants can and do vary in their sensitivity to growth regulator herbicides. In field demonstration studies, leaf cupping/distortion was observed when soybean plants were exposed to Banvel at a rate of 1/10,000 of the field use rate. Stage of plant growth when exposure occurs can also influence the amount of injury. Several studies in the literature report that soybeans were more tolerant to exposure to growth regulators when in early vegetative development as compared to exposure when the plants were larger and nearing the reproductive stage.
The herbicide most often discussed or implicated in the cupping response of soybeans is dicamba (see Table 1 for products containing dicamba). How would soybeans be exposed to this corn herbicide? Three possible avenues of exposure are as follows:
a. Residues remaining in/on the spray equipment from previous applications in cornfields are detached and applied with the soybean herbicide at low concentrations.
Labels of products containing dicamba provide techniques for cleaning application equipment to remove residues. The procedure from the label of Banvel states the following:
1) Hose down thoroughly the inside as well as outside surfaces of equipment while filling the spray tank half full of water. Flush by operating the sprayer until the system is purged of the rinse water.
2) Fill tank with water while adding one quart of household ammonia for every 25 gallons of water. Operate the pump to circulate the ammonia solution through the sprayer system for 15 to 20 minutes and discharge a small amount of the ammonia solution through the boom and nozzles. Let the solution stand for several hours, preferably overnight.
3) Flush the solution out of the spray tank through the boom.
4) Remove the nozzles and screens and flush the system with two full tanks of water.
(Even though Clarity contains the same active ingredient as Banvel, the spraying equipment cleaning procedure outlined on the Clarity label does not go into as much detail as the Banvel label does.)
If these cleaning procedures are not followed exactly, how much residue would remain in the application equipment, and would it be enough to cause injury to soybeans? Many producers and applicators who reported puckered soybeans in the past indicated that the symptoms appeared to follow the spray equipment "to the row."
Drift (discussed next) generally does not stop at a selected row in a field. Rather, there is often the feathering effect: symptoms are most severe on the side of the field closest to the source of drift and lessen with increasing distance. Unfortunately, failure to thoroughly clean the application equipment does not appear to explain the reported cases of ". . . the soybeans sprayed with the first load puckered, those sprayed with the second and third loads are fine, but the ones sprayed with the fourth load puckered" when all other factors are held relatively constant.
b. Herbicide vapors on the plant or soil surface move out of the treated area and are absorbed by soybeans (vapor drift).
The volatility of a herbicide is a function of several factors: those related to the formulation of the herbicide and those related to prevailing environmental conditions. Vapor pressure is a measure of the tendency of a herbicide to volatilize. As the vapor pressure of a herbicide increases, the potential for volatility also increases. Ester formulations of 2,4-D are generally much more volatile than amine formulations. Banvel is formulated as the dimethylamine salt of dicamba, Clarity as the diglycolamine salt, and Marksman as the potassium salt (see Table 1 for formulations of other dicamba-containing herbicides). Each of these salt formulations differs in its potential to volatilize. With respect to environmental conditions, volatility tends to increase as soil moisture and temperature increase. As soil moisture decreases, the amount of herbicide adsorbed to soil particles can increase and thus reduce the amount of herbicide available to volatilize.
c. Physical drift of spray particles during the actual application process.
This cause of exposure may be the easiest to identify based on field observations. The labels of many post-emergence herbicides have statements regarding wind speed and drift. Most specify that applications should not be made when wind speed is in excess of x miles per hour or moving toward a sensitive crop.
2. The soybean plant is expressing a physiological response to somewhat adverse growing conditions.
This theory generally attempts to exclude exposure to a growth regulating herbicide in the explanation. Rather, soybeans express puckered symptoms due to environmental factors adversely impacting their growth. Very few components in the puckered soybean "equation" have held consistent over the past several years except that the majority of the cases are not noticed or reported until after the first few days when air temperatures exceed 90°F. Soybeans may be entering into a phase of very rapid growth and development, and some speculate that this phase may disrupt the hormonal balance within the plant. This theory exists because there have been cases of puckered soybeans that had not been sprayed with any postemergence herbicide and no cornfields were nearby. There are, however, no data available in the literature to support this theory.
3. The response is induced by a postemergence herbicide application.
The majority of soybean samples received at the Plant Clinic demonstrating puckering were previously treated with a postemergence herbicide, usually a translocated herbicide such as Pursuit, Synchrony STS, Classic, FirstRate or Roundup Ultra, but in some instances a contact herbicide.
Many of these applications include spray additives such as oil concentrates (petroleum or vegetable base) and an ammonium nitrogen fertilizer (28% UAN or ammonium sulfate). How can these applications induce puckering? Some theoretical explanations include the following:
a. Translocated herbicides move into the apical meristem, the location of hormonal control, and disrupt the hormone balance of the plant. Following the disruption of hormonal balance, the plant exhibits some response such as leaf cupping or puckering.
b. The spray additives are able to remove dicamba residues from the spray equipment (see 1a above).
c. If 28% UAN was used, the level of biuret may be high enough to induce the response.
So what exactly is the cause of puckered soybeans? In short, no data exist that definitively explain every case of puckered soybeans. It is unlikely that one "blanket" explanation exists; each case may be somewhat unique. Data exist that describe the response of soybeans to exposure to growth regulator herbicides, but other factors may also be at work. If puckered soybean plants were actually exposed to a plant growth regulator herbicide such as dicamba, will yield be adversely affected? The available literature tends to suggest that this type of injury does not always necessarily result in soybean yield loss, but several factors are involved in determining if yield loss will occur. In particular, soybean variety, time of exposure, and dosage are important factors that determine whether yield loss will or will not occur. Much of the available literature suggests that if minor exposure occurs during early vegetative development, yield loss is less likely to occur than if exposure occurs when soybeans have entered the reproductive stage of development.--Aaron Hager and Christy Sprague